1. Field of the Invention
The present invention relates to a car phone antenna assembly, and more particularly to an antenna assembly which may be mounted on a window of a car and which can transmit and receive radio phone signals in the car phone band frequency range.
2. Description of the Prior Art
In U.S. Pat. No. 4,238,799 there is described a half-wave communications antenna assembly which can be mounted on a non-conductive surface, especially on a window of a vehicle. The above-referenced half-wave assembly is designed for use in the citizens band frequency range (26.965-27.405 MHz) as a mobile communication antenna. The half-wave antenna assembly includes a half-wave inductively loaded radiating whip loaded at its base end by a capacitor plate adapted to be affixed to a non-conductive surface on the vehicle. The capacitor plate is coupled to a coupling plate through the non-conductive surface and forms a coupling capacitor together with the capacitor plate. A tuned circuit is connected to the capacitor plate and is tuned to the resonant frequency of the antenna assembly. Thus, as compared with a ground plane antenna, the above referenced half-wave antenna does not need to be attached to a vehicle by clips or clamps, or by drilling a hole through the surface of the body, but can be mounted to the surface of a non-conductive body rather easily and rapidly.
But, in this type of a half-wave antenna assembly, the radiating whip is for use in the C.B. frequency band and, therefore, is to be continuously loaded by a helical, continuous coil extending about 22 inches and being comprised of copper wire in the form of a 1000 turn. Also, in the lower surface of the whip base acting as a capacitor plate, a tuning slug is slidably received within a channel of the base. The relative position of the tuning slug with respect to the base then allows for fine tuning of the resonant frequency of the half-wave antenna assembly by varying the surface area of the base. Accordingly, the structure of the base becomes complicated. A further disadvantage encountered in such a type of half-wave antenna assembly is that since a tuned circuit including a tapped coil and a capacitor is enclosed in a non-conductive, non-shielding tuning box cover which is retained on the coupling capacitor plate, the size of the tuning box cover must be enlarged to an undesirable degree.
On the other hand, an antenna assembly for cellular car phones is known in the art. This car phone antenna assembly can transmit and receive radio phone signals in the car phone band frequency range (821-896 MHz), and is mounted on a window glass of a car. The above referenced car phone antenna assembly comprises a stainless steel whip coated with black Teflon S, a whip base which allows angle adjustment of the whip, which is affixed to and mounted on a window of a car, and a coupling unit which is coupled to the whip base through the window and acts as an impedance matching circuit between the whip and a transmission cable. Further, a coupling capacitor disposed on the inside of the window just opposite to the whip base, a linear coil and a flat condenser member are also included in the coupling unit. The linear coil and the flat condenser member determines the resonant frequency of the antenna assembly and the facing distance between the flat condenser member and the coupling capacitor plate can be varied by a manual tuning screw installed in the coupling box for further fine tuning of the resonant frequency of the antenna assembly.
Nevertheless, this kind of car phone antenna assembly suffers from disadvantages that the flat condenser member may vibrate mechanically while a car is driven so that the resonant frequency determined by the linear coil and the flat condenser member may be changed slightly, which in turn reduces the gain of the antenna assembly and causes antenna noise to occur. A further disadvantage is that the size of the coupling box is enlarged, since the flat condenser member needs a large installation space.